Demand is ever increasing for AC-DC power adapters with high efficiency and high power density, especially where portable devices such as laptop computers are to be more affordable and portable. Compared with a flyback converter, LLC topology is gaining favor due to the proved higher efficiency. Previous studies of the LLC-based power adapter were focused on 90 W-130 W power design, in which two-stage configuration was prevailing to satisfy both Power Factor Correction (PFC) and the DC-DC conversion requirements. Due to improved semiconductor fabrication processes, CPUs now consume less power, and the design of power adapters has been reduced to 45 W-65 W. Within such power range, power factor is no longer a mandatory requirement, thus it is possible to remove the PFC stage.
For portable devices such as laptop computers, the AC-DC adapter is preferably designed to be a universal AC input adapted. Conventionally, a full bridge (FB) diode rectifier with a DC link capacitor is used as the rectification stage between the grid and the LLC converter. However, in order to operate from 90 V AC to 264 V AC, very high DC voltage gain is required for the LLC converter, and such design will degrade the efficiency. To reduce the required operational input voltage range, a hybrid full bridge voltage doubler (FB-VD) rectifier may be used (FIG. 1). The design uses the full bridge rectifier for 220 V AC input, while the voltage doubler rectifier is used for 110 V AC input However, the switch count is high and switch stress is high for conventional single stage FB-VD rectifiers as shown in FIG. 1. Also, the driving scheme is complicated.